(a) Field of the Invention
The present invention relates to a backlight unit for use in a liquid crystal display (LCD) unit and, more particularly, to a backlight unit having a reflector formed integrally with a reflection sheet for an optical guide plate.
(b) Description of the Related Art
LCD units are widely used in the field of televisions and cellular phones in addition to the fields of peripheral devices for computers. FIG. 8 shows an LCD unit having a conventional backlight unit, such as described in Patent Publication JP-A-2002-40413.
The conventional LCD unit includes an LCD panel 20′ wherein a liquid crystal layer is sandwiched between a pair of glass substrates, a backlight unit 10′ for generating light for the LCD panel 20′, an optical sheet 30′ disposed between the backlight unit 10′ and the LCD panel 20′ for providing the light from the backlight unit 10′ to the LCD panel 20′, and a housing (not shown) for receiving therein these members as a whole.
The backlight unit 10′ includes a rectangular optical guide plate 11′ having a front irradiation surface 11a, a tubular lamp 12′ extending along three edges of the rectangular optical guide plate 11′, a reflection member 43 for guiding the light from the lamp 12′ toward the irradiation surface 11a of the optical guide plate 11′, and backlight chassis 14′ for receiving therein the reflection member 43 and the optical guide plate 11′ for support thereof in unison. The reflection member 43 includes a reflection sheet 43b adhered onto the rear surface of the optical guide plate 11′, and a reflector 43a for encircling the lamp 12′ at three sides of the lamp 12′ including the front side, the rear side and one of the lateral sides far from the optical guide plate 11′. The reflection member 43 is of an integral type wherein the reflection sheet 43b and the reflector 43a are integrally formed.
The reflection member 43 of the integral type as described above is made of a sheet material, for example, obtained by covering polyethylene terephthalate (PET) with a reflection coat. FIGS. 9A and 9B show the shape of the sheet material and the final shape of the reflection member 43, respectively. A sheet material 48 having a thickness of 200 μm and shown in FIG. 9A is prepared, and folded along the pleat lines (shown by dotted lines) formed beforehand, to thereby obtain the final structure of the reflection member 43 including a reflection sheet 43b and a reflector, which are integrally formed. The pleat lines are formed on the sheet material 48 so that no cut portion of the pleat lines resides on the outer edge of the reflection member 43.
Back to FIG. 8, the backlight chassis 14′ includes a chassis body 14′a disposed between the LCD panel 20′ and the optical guide plate 11′ as well as between the LCD panel 20′ and the reflection member 43, and a chassis case 14′b for receiving therein the optical guide plate 11′ and the reflection member 43 in unison. The outer edge of the display area of the LCD panel 20′ resides in the very vicinity of and at the inner side of the inner edge of the chassis body 14′a. The reflector 43a of the reflection member 43 is positioned with respect to the optical guide plate 11′ by adhering the front edge of the reflector 43a onto the irradiation surface 11a of the optical guide plate 11′ via a adhesive tape.
The conventional backlight unit as described above has a problem in that the temperature change generated by the ON/OFF of the backlight source causes a lateral movement of the front side of the reflector 43a with respect to the optical guide plate 11′. The relative movement results from the reduction in the adhesive force of the adhesive tape due to the temperature rise, a difference in the heat capacity between the adhesive tape and the optical guide plate 11′, and the thermal expansion of the optical guide plate 11′.
More specifically, after the lamp 12′ is turned ON to raise the ambient temperature, for example, the adhesive tape first reduces in its adhesive force, and then the optical guide plate 11′ expands to shift the edge thereof toward the outer edge of the backlight chassis 14′. At this stage, the front side of the reflector 43a maintains its original position because the lateral side of the reflector 43a is supported by the chassis case 14b of the backlight chassis 14′. Thus, the edge of the optical guide plate 11′ moves toward the outer edge of the backlight chassis 14′ with respect to the front edge of the reflector 43a. 
After the ambient temperature falls, the adhesive tape recovers its original adhesive force, and then the optical guide plate 11′ contracts to recover the original state. This causes the front edge of the reflector 43a to be pulled by the edge of the optical guide plate 11′ via the adhesive tape, whereby the front edge of the lateral side of the reflector 43a departs from the chassis case 14b of the backlight chassis 14′ toward the central area of the LCD panel 20′.
Iterative cycle of the temperature rise and temperature fall causes a nonreversible gradual movement of the front edge of the reflector 43a toward the central area of the LCD panel, and the front edge eventually appears in the display area of the LCD panel 20′ to raise a defect on the screen.
For solving the above problem in the conventional backlight unit, the invention described in the publication uses a sheet material having separate front portions such as shown in FIG. 10. More specifically, the reflector 43a of the reflection member 43 has separate sheets 43c for the front side, formed separately from the reflector 43a and the reflection sheet 43b which are integrally formed. This configuration allows the front side of the reflector 43a and the optical guide plate 11′ to move in unison, upon the expansion and contraction of the optical guide plate 11′, thereby preventing the above defect.
In the conventional backlight unit described in the publication, although it is possible to prevent the above defect in the LCD unit, there arise other disadvantages as described hereinafter.
First, the separate sheets for the front side of the reflector impair the advantages of the refection member of the integral type, whereby the number of parts of the reflection member increases to increase the number of parts of the LCD unit and thus man-hours for fabricating the LCD unit.
Second, the separation of the front side from the lateral side in the reflector causes ingress of dust etc. through the gap therebetween, and increases the leakage of light to reduce the illumination efficiency of the backlight unit.